Laser radiation has been widely used in recent years for various treatment procedures including surgical cutting, vaporization of plaque in arteries, tissue ablation, coagulation, heating and tissue repair. In some cases, the laser radiation is applied externally, while in other cases the laser radiation is applied to a relatively inaccessible internal location. For internal application of laser radiation, laser catheters have been utilized. A conventional laser catheter includes an optical fiber which passes through a thin, flexible tube. The catheter is advanced through an artery or other body passage to a selected internal treatment location. Laser radiation from an external source is transmitted through the optical fiber to the selected internal location.
The selection of laser wavelength for a particular treatment depends on the requirements of the treatment, including depth of penetration, heating effects, treatment area and the like. Recent research in laser-tissue interactions has indicated the desirability of using mid-infrared wavelengths of about 3 micrometers for procedures such as ablation of myocardial tissue, vaporization of plaque in arteries, shallow coagulation and the like. The shallow penetration of these wavelengths allows clean holes, minimal trauma to the surrounding tissue and minimal particulate size. The most highly absorbed wavelength is approximately 2.94 micrometers which is, coincidentally, the exact output wavelength of the erbium-doped YAG laser. Although the erbium-doped YAG laser is easy to fabricate, the catheter delivery system for its output wavelength is not straightforward. The most commonly-used optical fiber is made of silica, which has a passband in the wavelength range of approximately 0.3 to 2.3 micrometers. Laser radiation outside this passband, including the mid-infrared range, is highly absorbed by silica and is not transmitted through the fiber.
It is known that zirconium fluoride optical fibers can be utilized for transmission of wavelengths in the mid-infrared band. However, fibers of this type have only recently become available and have numerous disadvantages including high cost, lack of mechanical strength, susceptibility to damage by high energy laser beams, a slightly hydroscopic nature and potential toxicity when the fiber is exposed to the human body.
A solid state laser having a miniaturized, quick-disconnect laser head is disclosed in U.S. Pat. No. 4,665,529 issued May 12, 1987 to Baer et al. Pumping radiation from a laser diode is carried through an optical fiber to the laser head. A neodymium YAG laser head, which has an output at 1.06 micrometers, is disclosed. The laser output is passed through a frequency doubler crystal. A high efficiency neodymium YAG laser pumped by a laser diode is disclosed in U.S. Pat. No. 4,653,056 issued Mar. 24, 1987 to Baer et al. In U.S. Pat. No. 4,538,278 issued Aug. 27, 1985 to Gergeley, a source of linearly polarized light at a wavelength of about 550-1100 nanometers supplies light through an optical fiber to a nonlinear crystal at the other end of the fiber. The crystal increases the frequency of the light from the source and provides light in the 250-550 nanometer wavelength range. Laser emission at 2.8 micrometers from an erbium-doped LiYF.sub.4 crystal is disclosed by G. J. Kintz et al in Appl. Phys. Lett. 50(22), June 1, 1987, pp. 1553-1555. The authors suggest pumping of the laser crystal with a laser diode array or with an alexandrite laser and suggest that the disclosed laser may have medical applications.
It is a general object of the present invention to provide improved methods and apparatus for medical treatment with laser radiation.
It is another object of the present invention to provide methods and apparatus for treatment of relatively inaccessible locations in the human body with laser radiation in the mid-infrared band.
It is a further object of the present invention to provide methods and apparatus for treatment of relatively inaccessible locations in the human body with radiation outside the passband of silica optical fibers.
It is yet another object of the present invention to provide a catheter having a laser at the distal end thereof.
It is still another object of the present invention to provide methods and apparatus for treatment of relatively inaccessible locations in the human body with laser radiation in the mid-infrared band while utilizing a silica fiber for transmission of laser radiation to the inaccessible location.